The invention relates to a method of growing mixed crystals having two lattice sites, each of which has a different number of adjoining oxygen ions from melts of oxidic multicomponent systems, and to the use of the mixed crystals manufactured in accordance with said method.
To obtain homogeneous crystals, the requirement has to be met that the distribution coefficient of the cations k.sub.eff(cat), i.e. the ratio of the concentration of cations in the solid phase to the concentration of cations in the liquid phase C.sub.s /C.sub.l is approximately 1. In this respect, it has to be taken into account that owing to the occurrence of analytical errors in the determination of the chemical composition of small concentrations of cations, distribution coefficients ranging from 0.90 to 1.20 are also included in the range of "approximately 1". Crystals are to be understood to mean herein both monocrystalline and polycrystalline materials.
Discs of single crystals drawn from melts of oxidic multicomponent systems are used, for example, as epitaxy substrates. In order to be able to grow flawless monocrystalline layers on monocrystalline substrates, it is required that the lattice constants of the substrate and the epitaxial layer correspond to each other as much as possible. For example, for the manufacture of monocrystalline superconducting films of, for example, YBa.sub.2 Cu.sub.3 O.sub.7-.delta. having lattice constants a=0.3818 nm, b=0.3886 nm and c=1.1680 mn, monocrystalline substrates of, for example, SrTiO.sub.3 (a=0.3909 nm), LaAlO.sub.3 (a=0.3792 nm) or LaGaO.sub.3 (a=0.3877 nm, b=0.3903 nm and c=0.5481 nm) are used. It is obvious that the lattice constants of all three substrates deviate clearly from the lattice constants of the superconducting film. Besides, microtwins are formed in the LaGaO.sub.3 and LaAlO.sub.3 crystals in the cooling phase, which microtwins have a strong negative effect on the crystal quality of the substrates. Consequently, such substrates are unsuitable for the manufacture of monocrystalline films of the superconductor. Thus, the simplest possible manner of manufacturing single crystals having the desired lattice constants has to be found.
A possible solution enabling the manufacture of substrates having the desired lattice constants consists in growing homogeneous mixed crystals whose lattice constant is adjusted through the crystal composition. It should be taken into account, however, that the distribution coefficients are of the cations k.sub.eff(cat) are in the range of 1, since only then homogeneous mixed crystals, i.e. mixed crystals having an approximately equal lattice constant throughout the length of the crystal drawn, obtained. Mixed crystals can be manufactured by mixing two terminal members of a series of mixed crystals; however, such mixed crystals have distribution coefficients which deviate relatively much from 1. Owing to the resulting low crystal quality and the change of the lattice constant in the longitudinal direction of the crystal, mixed crystals manufactured by mixing two terminal members are generally unsuitable for technical applications.
A possible solution could be the use of multicomponent systems whose melt compositions are used for growing homogeneous mixed crystals, because by virtue of the larger number of cations of said multicomponent systems they offer more possibilities of optimizing the distribution coefficients through the composition. So far, however, systematic approaches in this field are still in the stage of initial proposals.
In order to systematically produce mixed-crystal melt systems in which the distribution coefficients of all cations are close to 1, it is proposed in German Patent Application P 39 048 68.3 to use the average cation to oxygen bond lengths, i.e. structural data, to define such multicomponent systems because the insight has been gained that there is a direct relationship between these structural data and the crystal composition. The average cation to oxygen bond lengths can be calculated for mixed crystals having garnet, perovskite or spinel lattices by means of formulae which have been empirically determined for these lattices and which are known from scientific literature.